1. Field of the Invention
The invention relates to an electronic safety device including a switching element for disconnecting and/or for limiting the current consumption of an electrical load, and a method for operating the electronic safety device.
2. Description of the Related Art
Electrical safety devices are utilized wherever malfunctions within an electrical plant can endanger personnel and/or machines. The use of electronic safety devices is particularly customary in industrial plants. Here, a plurality of branch load circuits is frequently supplied by one power supply. Each branch load circuit contains one or more loads, where it is usually possible for each of these branch load circuits to be protected by an electronic safety device at the respective output of the power supply. In the event of a malfunction, currents through a branch load circuit are limited and in the case of a prolonged fault, the affected branch load circuit is disconnected. In this case, in comparison to a conventional fuse, the current limiting function of an electronic safety device offers the advantage that inrush currents of loads can be limited if necessary, even in regular operation.
With the increasing use of switched-mode power supplies as power supply units, with their precise current limiting and unambiguous detection of defective loads, electronic safety devices are being used in increasing numbers. In the event of a load short-circuit, the dip in the supply voltage is prevented by using an electrical safety device to actively limit the short-circuit current.
Apart from protecting each branch load circuit, there is often the need to temporarily disconnect individual branch load circuits. The reasons for this are, for example, maintenance operations or simply energy saving measures. In the majority of cases, however, these involve operational control of various loads (for example, solenoid valves, contactor coils or servomotors).
In the case of energy saving measures, individual branch load circuits are disconnected if the corresponding sections of the plant are not required. It should be noted here that the operating states of the entire plant and of the individual sections of the plant are always clearly defined to ensure a smooth start-up at any time.
In accordance with the prior art, relays and contactors are used to disconnect and connect individual branch load circuits. High inrush currents due to capacitive loads are capable of damaging or destroying a relay or contactor. It is therefore sensible to connect such a switching relay downstream of an electronic safety device. The electronic safety device then acts as a monitoring unit and operates to limit current surges in the course of a starting cycle. Here, the electronic safety device operates in a linear mode until the current again falls below a permissible maximum limit.
As a consequence of this inrush current limiting, lower-rated switching relays can be employed and the life of the switching contacts is significantly increased.